@MiaBedea When downloading, are you choosing the option to "Open with..." or to "Save file"? If your browser is trying to open the file, that can cause the error. Just save the files to your Downloads folder; you can later move them to Documents\Electronic Arts\The Sims 4\Mods manually.
If you download a file with the extension .zip, you can right-click and select Extract, which will unzip the file into something your game can read. Both Windows and macOS can extract .zip files without any additional software. For .rar and .7z files, you'll need a third-party app like 7-Zip (free download). If the files already have the extension .package or .ts4script, they don't need to be unzipped and can go directly into your Mods folder.
If you can't get your browser to just download the files without trying to open them, reset your browser preferences, or try a different browser.
